Without me, my keyboard will achieve nothing. Without a manager, a good developer will go on trying to create value.
The purpose of the manager is not to squeeze productivity out of a developer as you might blood out of a stone (although some seem to think it is), but to try to make sure that the developers are working on the right stuff for the company. In a healthy organisation, that can be done as part of a dialogue with the developers and takes the form primarily of communication.